Parylene is a generic name for a family of poly-p-xylylene polymers which may be deposited by CVD and polymerize by a free-radical mechanism that is self-initiating and non-terminating. The unterminated chain endings allow for unique chemistry to take place post-deposition. The monomer is typically generated by pyrolysis from a cyclophane or halogenated precursor.
The simplest commonly available polymer is parylene-N in which all carbon atoms are unsubstituted. Many other versions exist in which one or more of the carbon atoms are substituted with chlorine, fluorine or cyano groups, including parylene C, parylene D, parylene AF4, and cyanoparylene. Parylene polymers have a low dielectric constant (in the range of 2.3 to 2.6, e.g. AF-4 is 2.24 at 1 MHz out of plane) and have been tested as an interlayer dielectric in the fabrication of integrated circuits. Coating of parylene on wafers and other substances is known in the art. (See, for example, Plano et. al. Mat. Res. Soc. Symp. Proc. Vol. 476, pp. 231 (1997), Olson et. al. U.S. Pat. No. 5,709,753 and references cited therein).
Parylene coatings are typically obtained from sublimation of the cyclophane precursor followed by pyrolysis which forms the reactive intermediate, as shown in Scheme 1 for parylene N. The monomer in the vapor phase is fed into a deposition chamber where the monomer deposits on the surface of the substrate to be coated. Polymerization occurs at the surface, forming the parylene film.
Polymeric surfaces are notoriously hard to metallize and adhere to since they are relatively inert, possessing few surface reactive groups and a low surface free energy. To improve polymer metallization and polymer-polymer adhesion a myriad of processes have been developed to functionalize polymeric surfaces. These processes may be grouped as either dry, e.g. in a vacuum system, or solution-borne, as in with the use of wet chemistry. The dry methods typically use an arc discharge (capacitively coupled plasma) or a microwave plasma with various chemistries, i.e. H2O, O2, N2, N2O, NH3 and H2S. Surface modification of polymer surfaces has also been undertaken in vacuum by flowing H2O over a hot filament and by an Ar+ irradiation method.
Solution-borne processes typically use aggressive oxidizing reagents such as H2CrO4+H2SO4, KClO3+H2SO4, H2CrO4, fuming H2SO4, solvated electrons e.g. low temperature Mg/NH3, KMnO4, OsO4, NH3+AlCl3, or HNO3+H2SO4. The primary drawback of the solution-borne and the dry methods is their tendency to modify, not only the polymer surface, but also the underlying polymer subsurface and often inducing bond cleavage.
Accordingly, there is a need for gentle methods to improve metallization of the polymer substrate and/or adhesion between the polymer substrate and a higher energy surface or the surface of another polymer film.